Selectivity control system



Aug. 26, 1941. K RATH 2,253,942

SELECTIVITY CONTROL SYSTEM Filed April 24, 1940 3 Sheets-Sheet l INVENTOR i KRLR'TH ATTORNEY Aug. 26, 1941. K. RATH v I SELECTIVITY CONTROL SYSTEM Filed April 24, 1940 5 Sheets-Sheet 2 w .fl v n m b l 5 O 56 d I, FZ'AHPL/F/Ek :3 SECOND F NW0 mvENToR ATTORNEY Aug. 26, 1941. T

SELECTIVITY CONTROL SYSTEM s Sheets-Sheet 3 Filed April 24, 1940 ALA vvvv F/s."7f.

MFRYUQ/ICY INVENTOR ATORNEY Patented Aug. 26, 1941 SELECTIVITY CONTROL SYSTEM Karl Rath,

Patents Corporation,

York

New York, N. Y., assignor to Radio a corporation of New Application April 24, 1940, Serial No. 331,407

14 Claims.

The present invention relates to electric filter cient or frequency acceptance band of an electrical filter circuit or network in dependence upon 4 a controlling current or potential arises in many cases in practice in both high frequency and low frequency signalling systems. Thus, it is often desirable to vary the frequency response or transmission band width between the input and the output of an electrical system in a desired manner such as in the case of manual or automatic selectivity control in a radio receiver and for various other uses.

Accordingly, a basic object of the present invention is the provision in a transmission network including band-pass, low-pass, or highpass filters for transmitting electric wave energy, such as a modulated carrier signal in a radio receiver or the like, of means for effectively varying the electrical propagation factor or frequency band width purely electrically in proportion to variations of an electric controlling current or potential.

Another object is to provide an automatic fidelity control for a radio receiver of the type wherein the resonance curve of a tunable network is automatically varied in response to the variations in amplitude of the received signalling energy.

Still another object is to improve generally the efiiciency of radio receiving systems and particularly to provide automatic selectivity control in such a manner as to increase the frequency acceptance band and in turn the fidelity of the sound or other signals being transmitted in proportion to an increase of the signal strength, and to narrow the band width of the system for weaker signals, thereby to maintain optimum signal-to-noise ratio and varying interference and receiving conditions.

The above andfurther objects and advantages of the invention will become more apparent from the following detailed reference to the accompanying drawings forming part of this specification and wherein;

Figure 1 shows a band-pass filter incorporating an adjustable coupling according to the invention;

fidelity consistent with,

description taken with Figure 1a is a diagram showing the equivalent known filter network,

Figure 2 shows a modification of an adjustable filter of the type shown by the equivalent diagram in Figure 2a,

Figure 3 shows a further modification of the filter according to Figure 1,

Figures 4a and 4b show further modifications of an adjustable filter according to the invention and its known equivalent circuit, respectively.

Figure 5 illustrates an adjustable filter according to the invention embodied in a radio receiver for effecting an automatic selectivity control,

Figure 6 is a circuit diagram of a complete adjustable bandpass filter embodied inan amplifier such as the intermediate frequency amplifier in a radio receiver or the like, and

Figure 7 shows the variation of the resonance curve or frequency response characteristic in dependence upon the control potential obtained by a system according to Figure 6.

- Like reference numerals identify like parts throughout the different views of the drawings.

' With the above objects in view the invention involves the use of a variable space charge brought about in an electron discharge tube between a first accelerating electrode maintained at a relatively high positive potential with respect to the cathode and another decelerating electrode maintained at a' relatively low, including zero and negative, potential with respect to said first electrode and located at the side opposite from the cathode with respect to the first electrode. The first electrode may be the screen grid of a tube of standard construction having a plate maintained at cathode potential or biased negatively with respect to the cathode, in such a manner as to substantially prevent a steady electron current from being conveyed to said plate. If the electron current in such a tube is modulated in accordance with signal oscillations such as by impressing a signal potential upon a further control grid located near the cathode, a variable space charge also known as a virtual cathode will be established adjacent to the plate or other output electrode. This variable space charge or virtual cathode takes the form of a sheath of electrons of increased density spaced from the plate at a predetermined distance and being in electrical connection or coupling with the cathode and the other electrodes through the space current of the tube. The eilect of the variable space charge on the plate will be to induce a variable charge on the latter which will give rise to a plate current in an associated circuit t'ric' controlling current or potential.

of the same frequency as that of the oscillations impressed upon the control grid or other space current control element. The plate potential is displaced by 90 relative to the grid alternating voltage due to the capacitative induction by the variable space charge or virtual cathode in the manner described.

According to the invention a variable space charge coupling of the above type is employed as an effective coupling element between units or elements of a filter or transmission network for transmitting wave energy in a radio or other electrical communication system. v

"The space charge coupling and in 'turnthe propagation factor between an input and output circuit associated with the coupling tube is controlled in any suitable mannersuch as by'varying the steady bias on the input control grid, by: varying the screen grid voltage, by controlling the bias upon a special control electrode, by varying the decelerating potential on the plate or other output electrode or in any other suit- .able manner adapted to vary the distanceof the electron sheath from the plate and/ or its concentration or electron density. An explanation of the variable capacity effect obtained in applicants system in dependence upon a variable electric controlling magnitude such as a bias potential impressed upon a control grid or any other suitable control element, may be given as follows. As the grid potential becomes more positive the initial acceleration of electrons emitted by the cathode and attracted by the positive or screen grid will be increased thereby enabling suchelectrons to move to a point nearer to the plate before being repelled by the latter whereby the distance of the virtual cathode or electron sheath from the plate will be decreased resulting inan increase of the capacity in a manner Well understood. The opposite efiect takes place if the grid potential is made more negative causing a decrease of the'initial acceleration of the electrons. Although this explanation has been verified by tests and experiments made by applicant, the amount'of electrons or intensity of the space current was found to also play a part in the control and variation of the space charge coupling in addition to or in combination with the variation of the distance of the virtual cathode or electron sheath from the plate or other output electrode.

Referring to Figure 1, there is shown a bandpass filter comprising in a known manner a primary tuned circuit composed of an inductance HI shunted by condenser H and a secondary tuned circuit composed of an inductance 20 shunted by a condenser 2|. Electric wave energy such as modulated carrier signals are impressed upon the primary circuit Hlll by way of input terminals a"b and coupling coil l2 in inductive relation with the circuit ll I, and output energy is derived from terminals cd by way of coupling coil 22 in inductive relation With the circuit 20-2|. The primary and secondary circuits l0il and 20-2I are coupled with each other capacitatively as shown in the equivalent circuit of known type in Figure 10. by way of coupling condenser 26.

According to the present invention, the coupling condenser 25 is replaced by a variable space charge coupling constituted by an electron discharge tube I 3 and adapted to be readily controlled in proportion to the variation of an elec- The tube l3 in the example shown comprises a cathode M which may be either directly or indirectly heated, an input control grid I5, a screen grid [6 maintained in a known manner at a high positive potential with respect to the cathode and a plate IT. The primary circuit 19- in the example shown is coupled to the grid l5 and cathode M by way of a coupling condenser l8 and an adjustable resistance 23 forming a potentiometer together with a biasing battery 24 and serving to control the intensity or magnitude of the space charge coupling and in turn the coupling coeflicient between the circuits Iii-H and Ill-4|. Item 253is a by-passing condenser for the potentiometer resistance 23. The screen grid I 6 is advantageously by-passed to ground through, a condenser in a manner well understood, whereby the screen grid acts substantially as an accelerating electrode at ground alternating potential in producing a variable space charge or virtual cathode in accordance with the invention. v V 1 In the example shown the secondary circuit 2fi-. 2 is directly coupled between the plate I1 ,and cathode l4, whereby the plate I! is at cathode potential. If desirable, the plate may be biased negatively with respect to the cathode, such as by the provision of a suitable biasing source, as shown at I? in Figure 3, in such a manner as to substantially prevent a. steady stream of electrons emitted by the cathode and passing the openings of the positive grid [6 from being conveyed to the plate and to produce an intense variable space charge or dense electron sheath between the plate ll and the grid It. This space charge provides an eiiicient capacitative coupling between the circuits Ill-H and ,20- -2I. The latter may be tuned to the same frequency or they may be slightly d'etuned to obtain a desired band width characteristic. A further advantage of such a circuit in addition to the .possibility of efficiently controlling the band width or propagation factor by an electrical potential is the fact that the resonant circuits IIIH and 20-2l are coupled with each other substantially uni-laterally only; that is, in a direction from the input to theoutput of the tube l3, thus enabling an independent adjustment and balance of the system without mutual reaction as inthe case in the equivalent known cir .cuit employing a physical coupling condenser as shown in Figure 1A.

Referring to Figure 2, there is shown a filter network of the type according to Figure 2A with the coupling condensers 26 and 26' being replaced by space charge couplings in accordance with the invention. The resonant circuits [0-H and 20 2l in this example are coupled in push-pull to the cathodes 29 and 29 and input grids 30 and 307 ofa pair ofelectron discharge tubes 28 and 28', respectively. The tubes further comprise screen grids 3| and 31' maintained at suitable positive potentials with respect to the cathodes, and anodes or plates 32 and 32. The center points of the inductances H! and 2B are connected to ground or any other potential reference point of the system in the manner customary in push-pull systems. The cathodes 2e and 29' are connected to ground through a variable grid biasing resistance 33 by-passed by a condenser 34 serving to adjust the variable space charge between the screen grids 3! and 3i and the anodes 32 and 32', respectively, for effecting a proportionate variation of the coupling coefficient or by the provision of an additional control grid l6 located between the screen grid l6 and the plate I! and variably biased with respect to the cathode by the aid of a source 24 and potentiometer 23 in a manner similar to that shown in Figure 1. Item I4 is a biasing network in the cathodeto-ground lead to provide proper negative bias.

potential for the grid ito insure favorable operating conditions of the tube. If desired, an additional biasing source ll;' may be provided in the plate circuit. In practice a slight steady electron current may be conveyed to the plate which will not materially affect the function of the tube as a variable capacity or reactive energy transmission elementfrom its input to its output circuit.

Referring to Figure 4A, there is shown a further modification of a filter circuit of the general type shown in Figure 413 comprising a pair of condensers and 38 connected between the upper ends of the inductances l0 and 29, a pair of further condensers 31 and 39 connected between the lower ends of the inductances H) and 20 and a coupling condenser 5| placed between the junction points of the condensers 35, 38 and 31, 39. Figure 4A shows the equivalent circuit according to the invention embodying a space charge coupling in place of the condenser 5| constituted by an electron discharge tube 40 having a cathode 4|, a control grid 42, a screen grid 43,

and a plate 44. The grid 42 is connected to'the junction between the condensers 31 and 39 on the one hand and to the cathode through a grid lead resistance 46, a variable part of a potentiometer resistance 41 shunted by the biasing battery 48 and a further biasing resistance by-passed by a condenser and having its terminal opposite to the cathode grounded as shown, The potentiometer 41 is further by-passed by condenser 49.

The plate 44 is connected to the junction between condensers 38 and 38 on the one hand and to ground through a suitable impedance such as an ohmic resistance 50. There is thus provided a variable space charge coupling in place of 'a condenser 5| in Figure 43 whose magnitude can g:

be controlled in an easy and efficient manner such as by varying the grid bias in the manner illustrated to control the propagation constant or transmission characteristic.

Referring to Figure 5, there is shown. a radio receiver in block diagram form embodying an adjustable coupling network according to the invention to eifect an automatic selectivity or band width control. Radio signals received by an antenna are impressed upon a radio frequency f amplifier 51 by way of a coupling transformer 56 and converted into intermediate frequency signals in a first detector-mixer stage-58 in a man ner well understood by those skilled in the art. The intermediate frequency signals are selectively and efiiciently amplified in an intermediate frequency amplifier '59 including at least one stage embodying an adjustable coupling or band width control according to the invention which may be of any of the types described 'hereinbereaction between the circuits and increased fore. In the example shown this coupling circuit is substantially similar to'that according to Figure 1 wherein the input and output resonant circuits are tuned to the intermediate frequency of the receiver. The amplified intermediate frequency signals are impressed upon the second detector for demodulation, and the demodulated signals are further amplified in an audio amplifier 6| supplying an output for energizing a suitable translating device such as a loud speaker 82. A portion of the rectified energy is derived from the detector 63 by way of a choke coil 53 and a potentiometer resistance 64 which by proper filtering will supply in a known manner a potential varying proportionately'to the carrier amplitude of the radio frequency energy being received and serving in a known manner for automatic .volume control by controlling one or more variable mu high frequency and/or intermediate frequency amplifying stages. -In the example illustrated, this is indicated by the leads 66 and 61. A further potential derived from resistance 64 is impressed upon the grid I5 of the variable coupling stage in the, intermediate frequency amplifier in such a manner as to increase or decrease, respectively, the coupling effected by the variable space charge in proportion to an increase and decrease of the average amplitude of the received signal energy. In the example shown, if the signal strength increases the direct current through resistance 64 will increase thereby rendering the grid l5 more positive and increasing the variable space charge between the screen grid and plate of the coupling tube; that is, in turn increasing the coupling coefficient or band width of the control stage. Vice versa, if the signal strength decreases the grid [5 will become nega tive, thereby decreasing the coupling and narrowing the band width. In this manner the selectivity of the receiver will be automatically controlled to provide low selectivity or wide acceptance'band width for strong or local signals in which case noise and interference are substantially blanketed by the signal and to provide high selectivity or narrow band width in case of weak or distant signals and substantialelimination of noise and interference which without narrowing the acceptance band width would be increased considerably relative to the signal amplitude and in most cases render reception unpleasant and annoying, if not totally impossible.

The circuits l0ll and 28-2l are advantageously shielded from the remaining parts of the system such as by the aid of screens l0 and 20' resulting infurther prevention of mutual operational stability of the system.

Figure 6 is a complete diagram of a two-stage amplifier which may form a part of the intermediate frequency section of a superheterodyne receiver and embodies an adjustable band-pass filter or coupling network constructed in accordance with the invention. The input stage energized by way of terminal a-b is comprised of a pentode vacuum tube provided, with a biasing network 8| inserted in its cathode lead and a tuned output circuit 82, while the output stage is comprised of a pentode vacuum tube 82 provided. with a tuned input circuit 85, grid biasing network 81 inserted in its cathode lead and a resonant output circuit 88 in inductive relation with coupling inductance 89 connected to output terminal cd. The band-pass network interposed between'the circuits 82 and 86 comprises couplingelectron tube 8|, input resonant circuit 83' and output-resonant circuit 84 connected in accordance withthe invention in the'manner described hereinabove. Item 85 represents a gas discharge tube connecting the screen or positive grid of the coupling tube 81 to ground to insure a substantially steady positive potential and to stabilize the space charge coupling. The latter is controlled by a variable biasing potential provided by a suitable source 9| in conjunction with' adjustable voltage divider 9B and impressed upon the control grid and cathode of the coupling tub-e. 92 is a choke coil in the control lead to prevent alternating currents from entering the control source and 93 is a blocking condenser preventing short circuiting of the control potential.

According to experiments conducted by applicant it has been found that the effective band width or frequency response characteristic of an amplifier as shown in Figure 6 can be controlled in an easy and efiicient manner by varying the bias by adjusting the potentiometer 90 as illustrated in Figure 7. In the latter, the amplifier output is plotted as a function of frequency and curve A represents the selectivity or frequency response curve for arela-tively high negative potential on the input grid of the coupling tube (about 22 volts used in the tests) while curve B relates to a substantially de creased grid bias (somewhat less than zero according to the tests) showing a substantial widening or increase of the band width in accordance with the principle underlying this invention. As is -understood, the band width control potential for the coupling tube may be derived from any suitable source, such as an AVC network in a radio receiver to effect automatic band width control in a manner understood from the foregoing.

It will be evident from the foregoing that the invention is not limited to the specific circuits and arrangement of parts and details described and disclosed herein for illustration but that the underlying inventive idea and principle are susceptible of numerous modifications and *variations coming within the broader scope and spirit of the invention as defined by the appended claims. Thus, the circuits shown may be used as phase shifting systems to adjust the phase of an output potential derived from the output circuit of the coupling electron tube in relation to the phase of the input potential in proportion to the instantaneous magnitude of a controlling potential or current which may vary in accordance with a modulating signal to produce phase modulated output oscillations from a carrier of constant phase impressed upon the input of the tube. Many other uses will suggest themselves to those skilled in the art. The specification and drawings are accordingly to be regarded in-an illustrative rather than in a limiting sense.

I claim:

1. An electric filter for transmitting oscillatory energy comprising an input circuit and an output circuit, a reactive coupling between said circuits constituted by an electron discharge tube having at least a cathode, a control grid, a screen grid and an output electrode, means for maintaining said screen grid at a steady positive potential with respect to said cathode, means whereby said output electrode is maintained at a relatively low steady potential with respect to said screen grid to bring about a variable space charge adjacent to said output electrode, means for coupling said input circuit to said control grid and cathode, means for coupling said output circuit to said output electrode and cathode, a source of variable direct current 'bias potential, and further means for controlling said space charge in accordance with variations of said bias potential.

2. An electric filter for transmitting oscillatory energy comprising an input circuit and an output 'circuitfreactive coupling means between said circuits constituted by an electron discharge tube having at least a cathode, a control grid, a screen grid and a plate, means for maintain'ing saidscreen grid at a steady positive potential with respect to said cathode, further means for maintaining said plate at a relatively low steady potential with respect to said screen grid, to bring about a variable space charge adjacent to said plate, means for coupling said input circuit to said control grid and cathode, means for coupling said output circuit to said plate andcathojde, a source of variable direct current bias potential, and further means for controlling said space charge in accordance with variai-tons of said has potential. ,7

3. {in electric filter fortransmitting oscillatory energy comprising an input circuit and an output circuit, a reactive coupling between said circuits constituted by an electron discharge tube hav ing at least a cathodeapontrol grid, a screen grid and a plate, means for maintaining said screen grid at a steady positive potential with respect to said cathode, further means for maintaining said plate at a steady negative potential with respect to said cathode, means for coupling said input circuit to said controlgrid and cathode, further means for coupling said output circuit to said plate and cathode, a source of variable direct current bias potential, and means for controlling said space charge in accordance with variations of said-bias potential.

4. An electric filter for transmitting oscillatory energy comprising a pair of resonantcircuits tuned to a predetermined frequency, reactive coupling means between said circuits constituted by an electron discharge tube having at least a cathode, a control grid, a screen gridand an output electrode, means for maintaining said screen grid at a steady positive potential with respect to said cathode, further means for maintaining said output electrode at a relatively low steady potential with respect to said screen grid to bring about a variable space charge adjacent to said output electrode, means for coupling one of said resonant circuits to said control grid and cathode, further means for coupling the other resonant circuit to said output electrode and cathode, a source of variable direct current bias potential, and means for controlling said space charge in accordance with variations of said bias potential.

5. A band-pass filter comprising a first and a second resonant circuit tuned to a predetermined frequency, a reactive coupling between said resonant circuits constituted by an electron discharge tube having at least a cathode, a control grid, a screen grid and a plate, means for maintaining said screen grid at a steady positive potential with respect to said cathode, further means for maintaining said plate at a negative potential with respect to said cathode, means for coupling one of said resonant circuits to'said control grid and cathode, further means for coupling the other resonant circuit to said plate and cathode, a source of variable direct current aetae ia cordance with said bias potential. I

6; A band-passfilter comprising a pair ofresonant circuits, a reactive coupling'between said circuits constituted by anelectron discharge tube having at least a cathode, a control grid, a screen grid and an output electrode, means for mainpositive potential with respect'to 'said cathode, means whereby said output "electrode is main tained at a relatively low potential with respect to said screen grid to bring about a variable space charge adjacent to said output electrode, means 'for coupling one of said resonar'itcircuits to said control grid and cathode, 'furth'er means 'for coupling the, other resonant circuit to said output electrode and cathode, a source of variable direct'current bias potential, means to apply said bias potential to said control grid for controlling the coupling betweensaid circuits effected by said electron tube. I

7 7. A band-pass filter comprising a pairof res' onant circuits, a reactive coupling between said circuits constituted by an electron discharge tube having at least a cathode, a control grid, a screen grid,and a'plate, means for maintaining said screen grid at a steady positive potential with respect to the said cathode, further means for maintaining said plate at a negative potential with respect to said cathode, means for coupling one of said resonant circuits to said control grid and cathode, further means for coupling the other resonant circuit to said plate and cathode, a source of variable direct current bias potential, and means for applying said bias potential to said control grid to vary the effective coupling 'between said circuits effected by said electron tube.

8. The combination with a radio receiver, of a band-pass filter embodied in said receiver, saidhaving at least a cathode, a control grid, a screen grid and an output electrode, means for maintaining said screen grid at a steady positive potential wiith respect to said cathode, further means whereby said output electrode is maintained at a relatively low steady potential with respect to said screen grid to bring about a variable space charge adjacent to said output electrode, means for coupling one of said resonant circuits to said control grid and cathode, further means for coupling the other resonant circuit to said output electrode and cathode, means associated with said receiver for generating a control potential proportional to the carrier amplitude of the signal energy being received, and means for controlling said space charge in dependence upon said control potential to increase and decrease the coupling between said circuits effected by said electron tube in proportion to increasing and decreasing signal strength.

9. The combination with a superheterodyne radio receiver, of a band-pass filter embodied in the intermediate frequency section of said receiver, said band-pass filter comprising a first and a second tuned circuit resonant to the intermediate frequency, a reactivecoupling between said circuits constituted by an electron discharge tube having at least a cathode, a control grid, a screen grid and an output electrode, means for main taining said screen grid at a positive potential with respect to said cathode, further means taining said screen'grid at a relatively high steady ,whereby said output electrode is maintained at a relatively low steady potential with'respect to said sc'ree'ng'rid to bring about a variable space charge adjacent to said output electrode, means for coupling said first resonant circuit to said control grid and cathode, further means for coupling said'second resonant circuit to said output electrode and cathode, means associated with said receiver for generating a control potential proportional to the carrier amplitude of the signal energy being received, and means for controlling said space charge bysaid control potential to vary the coupling between said resonant circuits effected by said electron tube in proportion to variations of the signal strength.

10.;The combination with a superheterodyne radio'receiver, of a band-pass filter embodies in the intermediate frequency section of said receiver, said band-pass filter comprising a firs and a second resonant circuit tuned to the interme'diate frequency, a reactive coupling between said circuits constituted 'by an electron discharge tube having at least a cathode, acontrol grid, a screen grid and an'o'utput electrode, means for maintaining said'screengrid at a steady positive potential with respect to said cathode, further means whereby said output electrode is maintained at a relatively low steady potential with respect to said screen grid to bring about a variable space charge adjacent to said output electrode, means for coupling said first resonant circuit to said control grid and cathode, further means for coupling said resonant circuit to said output electrode and cathode, means associated with said receiver for generating a control potential proportional to the carrier amplitude of the signal energy being received, means for impressing said control potential upon said control grid to control the coupling between said resonant mediate frequency, a reactive coupling between said circuits being constituted by an electron discharge tube having .at least a cathode, a control grid, 2, screen grid and a plate, means for maintaining said screen grid at a steady positive potential with respect to said cathode, further means for maintaining said plate at a negative steady potential with respect to said cathode,

means for coupling said first resonant circuit to said control grid and cathode, further means for coupling said second resonant circuit to said plate and cathode, means associated with said receiver for generating a control potential proportional to the carrier amplitude of the signal energy being received, and means for applying said control potential to said control grid.

12. A system of the character described comprising a first and a second impedance means, a V

reactive coupling between said impedance means constituted by an electron discharge tube having at least a cathode, a control grid, a screen grid and an output electrode, means for maintaining said screen grid at a relatively high positive steady potential with respect to the cathode, further means for maintaining said output electrode at a negative potential with respect to said screen grid to bring about a variable space charge adjacent to said output electrode, means for cou-.

pling said first impedance means to said control grid and cathode, means for coupling said second impedance means to said output electrode and cathode, a source of variable direct current bias potential, and means for controlling said space charge in dependence upon variations of said bias potential to correspondingly vary the coupling between said first and second impedance means eflected by said electron tube.

13. An electric filter network for transmitting oscillatory energy comprising a first and a secend, a reactive coupling between said circuits constituted byan electron discharge tube having at least a cathode, a control grid, a screen grid and a plate, means for maintaining said screen grid at a steady positive potential with respect to said cathode, further means for maintaining said plate at a relatively low steady potential'with respect to said screen grid to substantially prevent a steady electron current from being con} veyed to said plate and to bring about a concentrated electron space charge adjacent to said plate, means for coupling saidfirst circuit to said control grid and cathode, further means for vcoupling said second circuit to said p late anem ode, a source of direct current bias potential, and means for controlling said space charge in accordance with variations of said bias potential to correspondingly vary the coupling between said first and second circuit efiected by said electron tube. 7

14. An electric filter network for transmitting oscillatory energy comprising a first and a second circuit, a reactive coupling between said circuits constituted by an electron discharge tube having at least a cathode, a control grid, a screen grid and an output electrode, meansior maintaining said screen grid at a steady positive potential with respect to said cathode, means whereby said output electrode is maintained at a relatively low steady potential with respect to said screen grid ,to substantially prevent a steady electron current frombeing conveyed to said output electrode and to bring about a variable space charge adjacent to said output electrode, means for coupling said first circuitto said control grid andcathodefme'ans for coupling said second circuit to said output electrode and cathode, a source of variable direct current bias potential, and further means for controlling said space charge in accordance with variations of said bias potential to correspondingly vary the coupling between 'said first and second circuit effected by said electron tube.

" KARLRA'I'H. 

